1. Field of the Invention
The present invention relates to an oxide semiconductor film, a method for evaluating an oxide semiconductor film, a semiconductor device, and a method for evaluating a semiconductor device.
Note that in this specification, a semiconductor device refers to any device that can function by utilizing semiconductor characteristics, and an electro-optical device, a semiconductor circuit, an electronic device, and the like are all semiconductor devices.
Further, the present invention relates to an object, a method, a manufacturing method, a process, a machine, manufacture, or a composition of matter. In particular, the present invention relates to, for example, a semiconductor film, a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a driving method thereof, or a production method thereof. Specifically, the present invention relates to, for example, a semiconductor device, a display device, or a light-emitting device including a transistor or a driving method thereof. Further, the present invention relates to, for example, an electronic device including the semiconductor device, the display device, or the light-emitting device.
2. Description of the Related Art
A technique for forming a transistor by using a semiconductor film formed over a substrate having an insulating surface has attracted attention. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit and a display device. A silicon film is known as a semiconductor film applicable to a transistor.
Whether an amorphous silicon film or a polycrystalline silicon film is used as a silicon film in a transistor depends on the purpose. For example, in the case of a transistor included in a large-sized display device, it is preferred to use an amorphous silicon film, which can be formed using the established technique for forming a film on a large-sized substrate. On the other hand, in the case of a transistor included in a high-performance display device where driver circuits are formed over the same substrate, it is preferred to use a polycrystalline silicon film, which can form a transistor having a high field-effect mobility. As a method for forming a polycrystalline silicon film, high-temperature heat treatment or laser light treatment which is performed on an amorphous silicon film has been known.
Further, in recent years, an oxide semiconductor film has attracted attention. For example, a transistor which includes an amorphous oxide semiconductor film containing indium, gallium, and zinc and having a carrier density less than 1018/cm3 is disclosed (see Patent Document 1).
An oxide semiconductor film can be formed by a sputtering method, and thus can be used for a transistor in a large-sized display device. Moreover, a transistor including an oxide semiconductor film has a high field-effect mobility; therefore, a high-performance display device where driver circuits are formed over the same substrate can be obtained. In addition, there is an advantage that capital investment can be reduced because part of production equipment for a transistor including an amorphous silicon film can be retrofitted and utilized.
An oxide semiconductor film is known to be sensitive to hydrogen and an oxygen vacancy (see Non-Patent Document 1). That is, it is important to control hydrogen and oxygen vacancies in an oxide semiconductor film in order that a transistor including the oxide semiconductor film has favorable switching characteristics.
However, since hydrogen is not a main component of an oxide semiconductor film, its evaluation requires an analysis method with high detection sensitivity. In addition, since hydrogen is a light element, its analysis method is limited. Therefore, it is sometimes difficult to evaluate hydrogen in an oxide semiconductor film.
Oxygen is a main component of an oxide semiconductor film. Thus, its analysis method is limited to a method capable of analyzing a main component. It is difficult to evaluate a minute change using a method suitable for analyzing a main component. Therefore, it is difficult to evaluate a minute increase or decrease in the amount of oxygen in an oxide semiconductor film. In other words, it is difficult to evaluate oxygen vacancies in an oxide semiconductor film.
As described above, it has been difficult to evaluate hydrogen and oxygen vacancies in an oxide semiconductor film. Therefore, it has been evaluated whether a transistor including an oxide semiconductor film has favorable switching characteristics by actually fabricating a transistor.
Note that low-temperature photoluminescence (PL) spectroscopy is disclosed as a method for evaluating defect states in an oxide semiconductor film (see Patent Document 2). According to Patent Document 2, a PL spectrum of an oxide semiconductor film which is obtained by low-temperature PL spectroscopy has a peak around 1.8 eV.